Ideas for wallet storage

The current way in which wallets are stored is a direct serialisation of the wallet class. This causes a couple of problems:

• the serialisation is quite brittle, small changes make the wallet unreadable
• because of the above I (Jim) have basically frozen the bitcoinj code by including it into MultiBit. This will cause problems as it is difficult to keep up to date with bitcoinj and contribute patches back.
• the serialisation is not readable by humans

In preparation of something better, I have put a wallet version number in the [info files](wiki/Wallet info files). This gives a hook to introducing a new wallet format whilst still supporting the old.